So last time we talked about constant volume and constant pressure scenarios for carrying out a reaction.
When we want to see how a reaction occurs outside the body (but resembling what happens inside the body), we used constant pressure systems. The difference is going to lie in the heat that comes out of doing the reaction in the scenarios.
The term enthalpy is used to talk about the change in heat that occurs in a constant-pressure reaction, and we measure it by the internal energy and PV (pressure * vol).
We talk about state functions, and how all this shit exists as a function of states...I just look at it by seeing what's up in the initial state vs. what's good in the final state of what you're doing in these reactions, whatever they may be. That's where all our Δ's come in, because we're looking at initial conditions vs. final conditions.
That ΔH value we get is what we got after we did all the math. When we measured the heat of reaction at constant pressure, we're actually measuring the change in enthalpy.
The book then goes on to start confusing me a little bit, since up to here I can sort of follow it. We then talk about how in most cases the difference between ΔU (internal energy change) and ΔH (change in enthalpy, or, change in heat that occurs in constant-pressure system) is practically negligible and to think of ΔH (enthalpy) as a measure of the energy change of a process. So enthalpy is change in energy? Google...help me out.
en·thal·py
ˈenˌTHalpē,enˈTHalpē/
noun
PHYSICS
- 1.a thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume.
Okay.
Now .... entropy
We use entropy to answer questions like that. Under a certain set of circumstances, some processes might want to go one way, and when you change the circumstances, they'll want to do something else. Like melting of water when it's warm and reforming of ice when it's cold.
But some things cannot go back and forth, they're irreversible. Once you introduce a certain set of circumstances to a certain thing, a reaction will take place, and you'll never be able to go back to your starting reactants.
Uhmm...this may be a bit morbid...but that's my cat. She used to be a cat. Then she died, and we sent her body to the crematory agency, and got back her ashes (and Dana helped me make a clay paw).
When you introduced the conditions of FIRE to CAT, you got ashes, and you can never have CAT back. How's that for irreversibly sad?
On the other hand, we have reversible processes, which will want to be near a state of equilibrium, being, the lowest possible energy state that a system can be in. Lower energy states of systems are favored over higher energy states, which is why systems want to be in lower energy states, therefore, they want to be at equilibrium. Irreversible shit is set up far from equilibrium, but it wants to be at equilibrium, so that's where it drives. I guess my dead cat is now at equilibrium.
In thermodynamics, favorable and spontaneous mean the same thing...it's just what the system wants to happen. But that doesn't mean that the favorable thing is going to happen quickly. It's just GOING to happen, we're not talking about rates of reactions, just what's happening IN them.
When you put water in the freezer, it's going to freeze. Just not right away. It's about what's favorable for the water in the freezer. Not how fast the water freezes.
Thermodynamics just has conversations with itself as to what is favored in a given situation.
Let's get a little philosophical and a little landmarky. I once asked my co-workers son what his favorite color was and why. He said: "blue is my favorite color because blue is my favorite color" and it reminded me of the difference between deciding and choosing as discussed in the landmark forum. But biochemistry isn't about choosing chocolate because it's chocolate, the favorable thing isn't the favorable thing because it's the favorable thing....there's more to it than that.
We can try to say that the favorable thing is the favorable thing because that's the lowest energy state of "thing". But minimal energy isn't going to explain everything. When ice melts, energy is absorbed.
So there has to be something else involved. The book talks about layering water on top of a solution of sucrose (a dissaccharide made of glucose & fructose). The stuff in the sugary solution is going to want to spread out throughout the newly added aqueous parts. There's not going to be changes in energy, no changes in heat, and no changes in work being done in the situation. The sugars are not going to decide to gather together and become a different layer. This is where this entropy crap comes in. Things are going to have a natural tendency to be random. And that's what entropy is, it's a "measure" of randomness. How much disorder there is in a system.
In terms of comparing energy states of systems (like let's say a bunch of legos), the entropy of an ordered state (like having them stacked up tightly to make a complicated wall) is lower than the entropy of a disordered state (having all the legos randomly dispersed throughout your living room).
Let's just, for now, say that shit wants to be random. That's where it's happiest. That's where you get high entropy.
We now can get into the second law of thermodynamics, stating that in an isolated system (where you put nothing in and nothing out), the entropy (amount of randomness) will increase to some...maximum.
Things aren't going to get more orderly on their own....not without...help
So...we don't really deal with isolated systems here. We deal with open systems. The world is a closed system but all the shit IN it is open systems. I put nutrients in the form of dog food and carrots into my cockroaches and they shit all that out as feces. Open systems :)
And in open systems, you need to account for both energy and entropy changes in the crap that happens to things living in the world, if you want to study it properly. That's how we ...somehow..get to Gibbs Free Energy, which is that thing that I kept saying that I don't believe in.
The thing Lenny talked about some, was that you cannot possibly directly measure this value. It's just a thing that's dependent on other crap that you can SORT OF measure, and then it gives you the free energy thing... ΔG...which is what we're going to use to explain if something is "favorable" or "unfavorable"
Free energy refers to the amount of ΔH that you've got that is available to be used to do work.
So what makes a process favorable?
Lower energy things, where ΔH is negative
Increases in entropy, where ΔS is positive
That's how you get favorable processes. Both of those situations will make ΔG negative which is what describes a favorable process.
This is a sentence from the book: "...for a favorable process in a nonisolated system, at constant temperature and pressure, is that ΔG be negative..."
THAT MEANS that if you have +ΔG, the process is NOT favorable, and the reverse of that process is favorable.
You then get words like exergonic and endergonic....
Exergonic processes are ΔG-
Endergonic processes are ΔG+
From the formula ΔG = ΔH - TΔS,
if ΔH and TΔS balance each other out, ΔG will be zero, meanings the processes is not favored in either direction, meaning the system is at equilibrium. The process is going to be reversible, and it can be pushed in going either direction by some external push.
In this next part, we're going to talk about how entropy and enthalpy play together when discussing liquid water/ice.
Here's how we're going to do this. We're going from ice -- > liquid and we have two different y-axes.
In ice, there's a certain number of bonds between molecules. When we melt ice, you're breaking some of those bonds. The difference in enthalpy between ice and liquid is similar to the energy you put into the system to break the bonds. So, the enthalpy change for ice --> liquid is positive:
Next, we want to consider what is happening with entropy in this system. There is a change in entropy because ice is more ordered than liquid. Water molecules get to play around doing their happy thing, while in ice, they are structured in an orderly fashion. So, when we go from ice to liquid, the entropy change is going to be positive because we're increasing randomness in the system.
This gives us some freedom to talk about gibbs free energy and the rest of the shit in its equation.
When we talk about ΔG for ice to water, when the temperature is low (and you have ice), ΔH (enthalpy) is going to dominate, meanings the ΔH term is going to be greater than the TΔS term, meaning ΔG is going to be positive....THAT means that since ΔG is POSITIVE, ice becoming water is NOT favored at low temperatures.
However, when we have higher temperatures, above the melting point of ice, the ice cube is going to melt.
ΔG is going to be NEGATIVE because higher temperatures, the TΔS value is going to be higher than the ΔH value when the T is large enough. So, at higher temperatures, the process of ice becoming water is favorable.
And up here, we can also see that at the melting point, 273 K, ΔH and TΔS are equivalent, and ΔG is going to be zero. This means the system is in equilibrium and any changes are reversible.
The melting point of something is the temperature where ΔH and TΔS lines intersect, which is the temperature where freezing is in balance with melting.
So when you're looking at reactions and you want to figure out what the favorable direction is, you're going to be looking at both entropy and enthalpy, and temperature is definitely also a factor.
Here's two important figures from the book:
This one has there processes. In the first one, both enthalpy and entropy work together and make the reaction favorable. In the second, the reaction is enthalpy favored (since ΔH is negative) but entropy opposes the reaction because entropy is a positive value...which you can say is bad for ΔG. However, ΔG still ends up being negative. Because enthalpy won, the reactionis enthalpy driven.
In the third process, you have an entropy driven situation. The enthalpy is a positive value, which is bad for ΔG, since it'll try to make ΔG positive, but we have enough entropy in the TΔS value (-140) to still have the reaction be favored and have ΔG be negative. That's why it's entropy driven. Because entropy won.
This is the second important figure. It's basically a summary. But you can't just fucking memorize this shit because it's going to make no sense if you don't understand it.
At low temperatures and high temperatures, the reaction is ONLY going to be favored if ΔG is negative. Having a positive ΔG makes the reaction not favored.
The rest of chapter three, I'm going to talk about in the next post.
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